Here is are a few brief descriptions of tools and projects that are using Elk as their extension language and that are present in the World Wide Web.
unroff: Scheme-based troff translator. unroff is a Scheme-based, programmable, extensible troff translator with a back-end for the Hypertext Markup Language. It is available for free as source and precompiled binaries for most major architectures. Unroff is not only useful by itself as a tool for hypertext authoring and conversion of troff documents, but also as a prototype for an extensible, hybrid application using Elk as its extension language.
Unroff has been used to generate the Hypertext version of the Elk documentation directly from the troff source.
If you need suggestions or inspiration for your own Scheme-based application, or if you are looking for code that you can reuse in your project, have a look at unroff.
Sced: Constraint Based Scene Design. Sced is a modelling program that allows you to edit objects in a virtual world using geometric constraints. Sced has a Scheme interface based on Elk.
AL: The Animation Language. The Animation Language is a programming language for modelling and animation, similar to Pixar's RenderMan. It is implemented as an extension to Scheme and is using Elk as the underlying Scheme engine.
The Ginger Project. Ginger is a an easy-to-use yet powerful interactive graph editor. It uses Elk as its extension language subsystem.
AUC Networking Extension. Someone at Aalbork University in Denmark has written a networking extension for Elk. The extension can be used to write TCP/IP networking applications in Scheme. It works both with BSD sockets and TLI. (Unfortunately, the URL seems to be out of order. I don't know if the documentation has moved to a different site, and if so, where it can be found now.)
EXTDesign++ Feature Modeller. EXTDesign++ is a feature-based modelling system developed at the Laboratory of Information Processing Science at the Helsinki University of Technology, Finland. The system is implemented in C++. Users of EXTDesign++ can enter, inspect, and manipulate feature definitions through a Scheme interface built around Elk.
A Signal Editor for the IRCAM Musical Workstation. The SignalEditor is an extensible set of tools for viewing, analyzing, editing, and processing sampled signals on the IRCAM Musical Workstation using various graphical representations. Elk is used by the SignalEditor to provide programmability and dynamic loading of extensions.